Is this lie worth fighting about?

My partner lied to me on friday night I've just found out. They told me they were told they had to be home by 9 and couldnt sleep over at my place because their parents had to visit their sick granda in hospital and they had to watch the dogs. They knew how important it was i seen them and how much planning i had put into friday and they lied to go to a party instead. We argued 2 days ago and i cant be bothered now things are back on track, how to i deal with this?